Imaging arrays for detecting electromagnetic radiation are well known in the art. For example, charge coupled device (CCD) imaging arrays are used in camera recording devices. Improved quantum efficiency, which results in improved response to low light levels, is a desirable performance feature which manufacturers attempt to optimize in such devices.
The quantum efficiency of an imaging array is directly affected by the transmissivity of the material through which the radiation must pass prior to entering the active absorber layer of the imaging device. For example, in a MOS imaging array, light must pass through the gate material prior to being absorbed in the active area of the substrate located below the gate. Polysilicon is used extensively for forming gates for visible imaging arrays even though visible radiation is inherently absorbed in the polysilicon material. To minimize the absorption loss, polysilicon layer thicknesses are used which are less than the absorption depth of most of the visible radiation spectrum. However, even with layers of polysilicon less than 5000 Angstroms thick, the quantum efficiency of such devices is significantly degraded. Transmission of the blue portion of the spectrum is especially poor.
For high performance applications, transparent gate materials have been developed, for example tin oxide and indium tin oxide. These materials are transparent throughout the visible spectrum, however, there are fabrication difficulties associated with their use. These materials require a dielectric layer to isolate adjacent gates, as shown in FIG. 1, where adjacent gates 1 through 5 are separated by dielectric layer 7. Imperfections in the deposited dielectric layer 7 reduce the yield of the fabrication process, since any pin holes will result in gate to gate shorts. The presence of dielectric layer 7 also creates a problem because it is not formed under all of the gates. In FIG. 1, gates 1, 3 and 5 are formed on a thermally grown gate dielectric layer 6 of silicon dioxide or silicon dioxide and silicon nitride. Gates 2 and 4, however, are formed over not only layer 6 but also the dielectric layer 7. Therefore, the drive voltage required for gates 2 and 4 is higher than that for gates 1, 3 and 5. This difference complicates the design and operation of the circuitry which interfaces with the device.
An alternative approach to improving the quantum efficiency of these high performance visible imagers is to thin the array's substrate after device fabrication, and to illuminate the array from the back side. The substrate is typically thinned to ten microns or less. This technique circumvents the gate absorption issue, but it presents difficulties in achieving reliable and uniform processes for thinning, for backside passivation and for connecting to the front surface.
The above materials and fabrication techniques have been known in the art for a decade or more. However, there has been a continued need for improved performance with photodetectors, particularly in low light applications, and for improved sensitivity in the blue portion of the visible spectrum. With the advent of high definition television, there is an even increased commercial demand for small, highly efficient detectors which can be produced at a reasonable cost. Existing technologies nave had difficulties satisfying this demand.